How to I fix a model I build crooked?
You can (and should) select half of the model even if it's not symmetrical. Unfortunately, once you activate the mirrors modifier the model will look like it's been torn up the middle.

To fix, enable clipping in the modifier panel, then select all of the vertices but the head. View your object from the front (numpad1), and hit R to rotate the crookedness away until all of the vertices are aligned straight up the middle.

You may need to do a little tweaking to get it to look right again, but the crookedness should be gone.

If your only blocking out a body, and it's a pretty low density mesh, it might be easier to delete it and start again.